Selective electrochemical detectors have numerous applications in diverse technical areas. For example, such detectors can be used to detect inorganic combustion products in the exhaust gas of natural gas combusters, internal combustion engines, and various types of heating furnaces and boilers. Electrochemical detectors also have applications for detecting and quantifying the concentration of various components in chemical plant or petrochemical refinery processes. Furthermore, such detectors have medical applications, for example, detecting anesthetic gas (laughing gas) or measuring oxygen levels in blood. The detectors can be used for automated process control and for continuous monitoring of environmental pollutants.
In particular, the determination of nitric oxide is very important for many different reasons. Nitric oxide is toxic at high concentrations, and its release into the atmosphere causes many environmental concerns. It can also be valuable to monitor nitric oxide in the exhaust stream produced by the combustion of organic fuels.
One of the most common types of detectors for nitric oxide in the exhaust gas of natural gas combusters, internal combustion engines, and various types of heating furnaces employs chemiluminescence of excited states generated by the reaction of nitric oxide with oxygen, hydrocarbons, or carbon oxides on a catalytic surface. However, these chemiluminescence detectors have long term stability problems and a rather complicated electronics system. Thus, there is a need for a more stable and less complicated detection system for nitric oxide in the exhaust gas from a combustion process.
Another type of nitric oxide detector is of semiconductive type. Semiconductive gas sensors employ a polycrystalline-oxide semiconductor material that is coated with porous metal electrodes to form a semiconductor "sandwich." The semiconductor material is typically formed, for example, of SnO.sub.2 or ZnO. The porous electrodes are typically formed of platinum and are used for measuring the conductivity of the semiconductor material. When gases, such as oxygen (O.sub.2), nitric oxide (NO), nitrogen dioxide (NO.sub.2), carbon monoxide (CO), and methane (CH.sub.4, natural gas) are absorbed on the surface of a semiconductor material, the electrons in the conduction band of the polycrystalline oxide are pinned in O.sup.2- surface states, which changes the conductivity of the semiconductor material. The surface conductivity of each semiconductor particle is slightly reduced through an interparticle barrier potential that depends upon the partial pressure of oxygen or other electron accepting gas absorbed on the semiconductor material. V. Lantto, R. Pomppainen, and S. Leppavouri, Sensors and Actuators, 14, 149 (1988). It appears that the sensitivity of the detector depends upon the enthalpy of the gas absorption onto the surface of the semiconductor, which determines coverage as a function of gas partial pressure and the ease of electron transfer to the gas molecule.
Although semiconductive gas sensors often have good sensitivity and fast response time, this type of detector is normally limited to operation at temperatures below about 300.degree. C. and usually does not show long-term stability. The limitation to relatively low operating temperatures makes this type of detector susceptible to extraneous effects that include "memory effects" and temperature dependent water absorption. The memory effects are a function of the reversibility of the absorption of the gas particle on the surface of the semiconductor material, which typically requires that the semiconductive gas sensor to be operated at above 200.degree. C. to achieve complete reversibility of absorption.
Furthermore, the gas sensors of the semiconductive type are rarely selective to particular chemical species. However, at least one semiconductor material, In.sub.2 O.sub.3 --SnO.sub.2 (9:1 ratio by weight), has been reported to be selective to nitrogen dioxide (NO.sub.2) at temperatures of 200.degree.-350.degree. C. G. Sberveglierei, S. Groppelli, and G. Coccoli, Sensors and Actuators, 15, 235 (1988). Recently, a Cr.sub.2 O.sub.3 --Nb.sub.5 O.sub.5 semiconductive type detector with high selectivity for nitric oxide (NO) in the presence of nitrogen dioxide, carbon monoxide, and carbon dioxide has been reported. T. Ishihara, K. Shiokawa, K. Eguchi, and H. Arai, Chem, Lett., 997 (1988). However, the selectivity of this detector maximizes at 200.degree. C. and disappears completely at 500.degree. C. The loss of selectivity at high temperatures is induced by desorption and reduction of the semiconductor surface. In general, the strong temperature dependence of the selectivity and the small conductivity changes induced by the surface adsorption process have limited the use of this type of detector for quantitative measurements of nitric oxide.
A modified electrochemical oxygen detector has also been reported for the detection of nitric oxide in a gaseous mixture. One of the most accurate oxygen sensors known in the art is of the limiting current type, which employ the principle of an electrochemical pumping cell. E. M. Logothetis and R. E. Hetrick, in "Fundamentals and Applications of Chemical Sensors", Chapter 4, ACS Symposium Series, 309, 71 (1986).
In a typical oxygen sensor of the limiting current type, yttrium (Y.sub.2 O.sub.3) stabilized zirconium (ZrO.sub.2) is used in an oxygen pumping cell as a solid electrolyte that allows for mobile oxygen anions. Oxygen enters the pumping cell through a porous "front" electrode that is polarized so that oxygen is reduced to O.sup.2- moves through the electrolyte in this form, and then is oxidized back to O.sub.2 at the porous "back" electrode. The rate at which oxygen is pumped from the front electrode to the back electrode increases with increasing applied voltage across the electrodes.
A diffusion barrier is positioned between the analyte gas and the porous front electrode of the oxygen pumping cell. Diffusion barriers can be of the aperture type, the porous media type, or combinations thereof. Porous media include ceramics, micromachined materials, and polymers.
The diffusion rate of oxygen through the diffusion barrier from the analyte gas to the "front" electrode of the pumping cell is determined by the overall geometry of the detector, the diffusion constant of the barrier, and the partial pressure of oxygen in the gas. The diffusion constant for a particular diffusion barrier and a particular chemical species is further dependent on a plurality of factors, including temperature, However, at a constant temperature of operation, the diffusion rate is essentially proportional to the partial pressure of oxygen in the analyte gas.
The ionic oxide current in the oxygen pumping cell increases with applied voltage until complete oxygen depletion occurs at the "front" electrode. To obtain a limiting current, the oxygen pumping cell of the electrochemical detector must be able to pump oxygen from the front electrode through the electrolyte to the back electrode at a faster rate than oxygen can diffuse through the diffusion barrier to the front electrode. Thus, the limiting current type of detectors typically operate in temperature ranges above 400.degree. C. where the ionic conductivity through the electrolyte is high. The value of the limiting current will depend upon the rate of diffusion of oxygen through the diffusion barrier to the front electrode, which under constant temperature operating conditions is proportional to the partial pressure of oxygen in the analyte gas. Thus, the limiting current is proportional to the partial pressure of oxygen in the analyte gas.
A more advanced form of the ionic current sensor involves two electrochemical cells, one for sensing and one for ion pumping. In this configuration, the ion pump is used to deplete a substantially closed space of oxygen, setting up an oxygen partial pressure difference between the front and back surface of the sensing electrode that acts as a Nernst concentration cell. The current-voltage relationship for the double cell depends on the external oxygen partial pressure and on diffusion and geometric characteristics as before but with advantages over the single cell configuration, which include: less temperature dependence; lower current and voltage operation (less electrode polarization and decomposition); and operation in any region of the current-voltage curve.
The recent modification of the double cell oxygen detector for determining the concentration of NO.sub.x employs a unibody construction made from yttria stabilized zirconia that consists of four cells. M. Noda, N. Kato, and H. Kurachi, European Patent Application No. EP 87306846.4, Publication No. 257,842, Mar. 3, 1988. One such double cell consists of a conventional oxygen pumping and oxygen sensing arrangement as discussed above, while the other double cell also consists of pumping electrodes and sensing electrodes, but with one of the platinum electrodes coated with rhodium. The rhodium will catalytically decompose nitric oxide (NO) and nitrogen dioxide (NO.sub.2 into nitrogen (N.sub.2 and oxygen (O.sub.2) at the operating temperature of 500.degree. C. and increases the partial pressure of oxygen detected at the sensing electrodes versus that detected at the oxygen sensor. The reference electrode part of both sensing elements is exposed to a constant reference pressure of oxygen. In this four cell arrangement the oxygen produced by selective decomposition of the oxides of nitrogen is read directly as a sensing voltage difference between Nernst cells. Thus, NO.sub.x can be selectively detected with the oxygen content of the gas stream internally compensated. However, the sensor cannot distinguish between nitric oxide (NO) and nitrogen dioxide (NO.sub.2). Moreover, the platinum electrodes of a common oxygen sensor are capable of a catalytic function with respect to other reductive gas components, such as carbon monoxide (CO). In the case where the measurement gas contains one or more reductive components (including CO, for example), the operation of this detector requires that the concentration of the reductive components, which affect the oxygen partial pressure, be known. Thus, the detector cannot be considered to have much selectivity. These are severe limitations on the usefulness of this type of detector.
Thus, there is a long-felt need for a stable detector that is highly selective to nitric oxide and capable of operating over a wide temperature range. For gas sensing applications, there is further a long-felt need for a stable detector capable of operating at elevated temperatures, particularly at temperatures above 300.degree. C. There is also a need for a low cost, relatively uncomplicated nitric oxide detection system.